1. Field of the Invention
The present invention generally relates to a thermal printer apparatus equipped with a plurality of heating elements arranged on a thermal printing head and capable of printing out on print paper by supplying energizing pulses to these heating elements in response to a print signal. More specifically, the present invention is directed to such a thermal printer apparatus capable of eliminating voltage variations and printing speed changes with employment of a simple structure.
2. Description of the Related Art
Conventionally, thermal printer apparatuses equipped with a plurality of heating elements arranged on thermal heads are utilized to print out characters and the like in such a manner that a printing operation is carried out on a print paper in a direct thermal manner, or a thermal transfer manner by applying energizing pulses to these heating elements in response to printing signals.
In general, such a thermal printer apparatus has a specific characteristic that remaining heat produced when these heating elements are energized may influence printing density. As a result, in order to keep the printing density constant, the conventional thermal printer apparatuses employ an historical controlling system. That is, in this historical controlling system, the energizing pulses are applied to the respective heating elements, while considering the drive histories of the respective heating elements.
For example, in the thermal printer apparatus described in Japanese Registered Patent No. 2681004, the energizing pulse width of the driven dot of interest is controlled by considering the voltage application time, the formula, and the drive histories of 4 sets of dots located near the above-described driven dot of interest. This voltage application time is defined based upon the peripheral temperature of the thermal head. The formula is determined by the printing cycle time and the like.
Furthermore, in the thermal printer apparatus disclosed in Japanese Registered Patent No. 2647062, the energizing time of the dot of interest is subdivided into the main energizing time and the sub-energizing time. The summation between the main energizing time and the sub-energizing time is determined based upon the peripheral temperature of the thermal head. Then, both the main energizing time and the sub-energizing time corresponding to the printing conditions acquired until 2 previous printing cycles have been accomplished are determined by the ratio fixed by the printing mode. As a consequence, the dot of interest is energized based upon these determined factors.
On the other hand, the thermal printer apparatus described in Japanese Registered Patent No. 2681004 has the problem that since the energizing pulse is determined based on both the peripheral temperature of the thermal head and also the printing cycle time, this energizing pulse cannot follow the variations contained in the voltages applied to the thermal head. Also, since a complex calculation must be carried out in order to produce the historical data, this thermal printer apparatus has another problem in that high speed calculating apparatus is necessarily employed, resulting in very expensive thermal printer apparatus.
Also, in the thermal printer apparatus described in Japanese Registered Patent No. 2647062, the summation between the main-energizing time and sub-energizing time is determined based only upon the peripheral temperature of the thermal head. Similar to the first-mentioned thermal printer apparatus, this thermal printer apparatus has the problem that the energizing pulse cannot follow the voltage variations. Furthermore, both the main energizing time and the sub-energizing time are subdivided by the ratio determined by the printing mode in this thermal printer apparatus. As the result, there is a further problem that this energizing pulse cannot follow a change in the printing speeds. In addition, since a specific gate circuit capable of outputting the sub-energizing pulse is required, the structure of this thermal printer apparatus becomes very complex, and eventually, this thermal printer apparatus is made very expensive.
The present invention has been made to solve the above-described various problems of the prior art printers, and therefore, an object of the invention is to provide a thermal printer apparatus having a simple arrangement, capable of achieving a sufficiently high printing quality, while suppressing adverse influences caused by a voltage variation and a change in printing speeds.
To achieve the above-described object, a thermal printer apparatus, according to the present invention, is characterized in that the thermal printer apparatus is equipped with a plurality of heating elements arranged on a thermal head, and an energizing pulse responding to a printing signal is applied to the respective heating elements so as to energize the heating elements, whereby a printing operation is performed on a print paper, the thermal printer apparatus comprising: temperature measuring means for measuring a temperature of the thermal head; voltage measuring means for measuring a voltage applied to the thermal head; and head drive means for calculating a maximum energizing pulse width based upon the measured temperature of the thermal head and the applied voltage thereto, for calculating a main pulse width based on both the maximum energizing pulse width and a printing speed, for calculating a preselected ratio of a sub-pulse width with respect to the maximum energizing pulse width, and for outputting to the thermal head, such an energizing pulse having a smaller pulse width selected from a summed width between the main pulse width and the sub-pulse width, and also the maximum energizing pulse so as to drive the thermal head.
In this thermal printer apparatus, the temperature measuring means measures the temperature of the thermal head, and the voltage measuring means measures the voltage applied to the thermal head. The head drive means calculates the maximum energizing pulse width based upon the measured temperature of the thermal head and the applied voltage thereto, calculates the main pulse width based on both the maximum energizing pulse width and the printing speed, calculates the preselected ratio of the sub-pulse width with respect to the maximum energizing pulse width, and outputs to the thermal head, such an energizing pulse having the smaller pulse width selected from the summed width between the main pulse width and the sub-pulse width, and also the maximum energizing pulse so as to drive the thermal head.
Also, the above-described head drive means includes maximum energizing pulse width calculating means for calculating the maximum energizing pulse width based upon both the measured temperature of the thermal head and the applied voltage; energizing pulse width calculating means for calculating the main pulse width based on both the maximum energizing pulse width and the printing speed, and also for calculating the preselected ratio of the sub-pulse width with respect to the maximum energizing pulse; and output means for outputting to the thermal head, such an energizing pulse having the smaller pulse width selected from the summed width between the main pulse width and the sub-pulse width, and also the maximum energizing pulse.
Also, the above-explained energizing pulse width calculating means may be arranged to calculate the sub-pulse width by multiplying the maximum energizing pulse width by a predetermined coefficient xe2x80x9cKxe2x80x9d.
Further, the above-mentioned coefficient xe2x80x9cKxe2x80x9d may be selected from a value within a range between 0.1 and 0.3.
Furthermore, the thermal printer apparatus may be arranged in such a manner that the sub-pulse is not applied to the heating elements which are energized at the prestage, and further are presently energized.